Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 97.6		$ 97.6		$ 107.9
Recorded Investment With No Allowance	41.5		41.5		51.4
Recorded Investment With Allowance	40.2		40.2		31.8
Total Recorded Investment	81.7		81.7		83.2
Related Allowance	10.5		10.5		10.8
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	79.0	$ 97.5	82.5	$ 101.0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.2	0.2	0.2	0.4
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	38.2		38.2		45.6
Recorded Investment With No Allowance	13.0		13.0		20.9
Recorded Investment With Allowance	15.6		15.6		14.1
Total Recorded Investment	28.6		28.6		35.0
Related Allowance	3.3		3.3		3.9
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	5.8		5.8		10.0
Recorded Investment With No Allowance	1.7		1.7		3.4
Recorded Investment With Allowance	3.3		3.3		0.5
Total Recorded Investment	5.0		5.0		3.9
Related Allowance	0.2		0.2		0.0
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	0.6		0.6		1.8
Recorded Investment With No Allowance	0.5		0.5		1.7
Recorded Investment With Allowance	0.3		0.3		0.0
Total Recorded Investment	0.8		0.8		1.7
Related Allowance	0.0		0.0		0.0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.4		4.4		4.7
Recorded Investment With No Allowance	0.2		0.2		0.4
Recorded Investment With Allowance	3.4		3.4		3.5
Total Recorded Investment	3.6		3.6		3.9
Related Allowance	2.1		2.1		2.2
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	10.8		10.8		16.5
Recorded Investment With No Allowance	2.4		2.4		5.5
Recorded Investment With Allowance	7.0		7.0		4.0
Total Recorded Investment	9.4		9.4		9.5
Related Allowance	2.3		2.3		2.2
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	9.2		9.2		11.5
Recorded Investment With No Allowance	6.1		6.1		8.2
Recorded Investment With Allowance	1.8		1.8		2.0
Total Recorded Investment	7.9		7.9		10.2
Related Allowance	0.3		0.3		0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	13.2		13.2		3.7
Recorded Investment With No Allowance	13.0		13.0		3.6
Recorded Investment With Allowance	0.0		0.0		0.0
Total Recorded Investment	13.0		13.0		3.6
Related Allowance	0.0		0.0		0.0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	71.4		71.4		77.3
Recorded Investment With No Allowance	34.5		34.5		38.2
Recorded Investment With Allowance	24.4		24.4		20.1
Total Recorded Investment	58.9		58.9		58.3
Related Allowance	5.9		5.9		6.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	56.4	62.9	58.6	65.5
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.1	0.1	0.1	0.3
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	24.4		24.4		29.5
Recorded Investment With No Allowance	5.7		5.7		12.4
Recorded Investment With Allowance	15.4		15.4		11.4
Total Recorded Investment	21.1		21.1		23.8
Related Allowance	4.4		4.4		4.4
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	20.7	32.5	22.5	32.2
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	0.1	0.1	0.1	0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.8		1.8		1.1
Recorded Investment With No Allowance	1.3		1.3		0.8
Recorded Investment With Allowance	0.4		0.4		0.3
Total Recorded Investment	1.7		1.7		1.1
Related Allowance	0.2		0.2		$ 0.2
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Average Recorded Investment	1.9	2.1	1.4	3.3
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Income Recognized	$ 0.0	$ 0.0	$ 0.0	$ 0.0